1. Field of the Invention
Embodiments of the present invention generally relate to the marine storage of liquefied natural gas, and more particularly, to the design and construction of marine storage tanks that possess strength and stability against loads caused by the stored fluids and the environment.
2. Description of Related Art
Clean burning natural gas has become the fuel of choice in many industrial and consumer markets around the industrialized world. When natural gas sources are located in remote locations, relative to the commercial markets desiring the natural gas, a mechanism to transport the natural gas to market is needed. One such mechanism may include transporting the natural gas through pipelines in gaseous form or may include transporting the natural gas in liquid form via large-volume marine vessels.
Vessels designed to carry liquefied natural gas (LNG) may involve large capital expense in comparison to other cargo carrying systems. This may be in part due to the cryogenic temperature required to maintain LNG in a liquid state under near ambient pressure for long sea-transit. Because LNG is relatively light, a vessel may have a larger volume capacity for a given weight of cargo, as compared to other types of cargo.
One of the challenges for LNG storage tank design may be to ensure that the LNG storage tanks have enough structural integrity to withstand loads due to cargo motion and sloshing. Sloshing is liquid motion within a tank that may be produced by periodic motions (e.g., ships at sea). As the liquid in a tank moves waves may be formed and waves traveling in a fluid contained within a tank of fixed length may interfere with waves that have reflected off the end of the tank and are traveling back in the opposite direction. At certain frequencies, standing waves can be produced and may be a resonance phenomenon. The frequency at which standing waves occur may be called resonant frequencies. When the frequency at which force is applied is near the resonant frequency of the fluid within the tank, large increases in amplitude may occur, possibly resulting in large forces being exerted on the tank.
Sloshing may be a concern for vessels that carry liquids in their storage tanks and may be considered during the design of such ships. Sloshing may become more pronounced when the frequencies of ship motions match frequencies associated with the liquid motion in the storage tanks. The frequencies that may be associated with the liquid motion in the storage tanks may be functions of the tank geometry and the cargo fill levels in the storage tanks.
The sloshing of fluids may result in various problems with the vessel and/or storage tanks. For instance, sloshing related damage to the structure of storage tanks may be the result of a single large load event, or cumulative events. Cumulative damage may be the result of a large number of smaller load events, which combine to progressively degrade the structure of the storage tank, a membrane inside the storage tank and/or an insulation system used to maintain the temperature of the storage tank. Further, sloshing of fluids, such as LNG, can be problematic because it may increase the hydrodynamic loads on a marine vessel's hull structure. Also, sloshing may reduce the stability of the vessel and may promote vaporization of the LNG in the storage tanks.
Accordingly, in determining the type of storage tank to use, sloshing and other limitations have to be considered. For instance, free-standing tanks, such as spherical and prismatic tanks, may provide access to the containment system and hull of the vessel. However, free-standing tanks may require plates, which are thick, heavy and expensive. As a specific example, spherical tanks may have a wall thickness ranging from about 30-60 millimeters (mm), which may add weight and increase cost relative to other storage tanks. Further, the shape of spherical tanks may not match the available space on a vessel, which may result in upper portions of the spherical tanks extending about 15 meters (m) above the main deck. This extension may increase the height of the vessel's center of gravity. The increase in the center of gravity for the vessel may increase the vessel's vulnerability to weather effects (e.g., wind and icing), and require an elevated aft bridge to provide visibility over the spherical tanks. To permit loading form the top, as may be required by regulation, considerable access structures (e.g., ladders, catwalks and piping) may also be added above the deck of vessels fitted with spherical tanks. In addition, some free-standing tanks, such as prismatic tanks, may also require extensive bracing to overcome the loads due to the cargo and the weight of the tank itself.
Further, while avoiding some drawbacks of free-standing storage tanks, particularly in weight and material cost, prismatic membrane tanks may also limit access to the vessel. For instance, the prismatic membrane tanks may limit access to the interior of a vessel's inner hull and the exterior of a storage tank's insulation and secondary barrier.
Accordingly, there is a need for a method to design storage tanks, which may include LNG, CO2, and other fluids, that are configured to store refrigerated/cryogenic fluids and provide suitable strength and stability against movement (e.g. sloshing) of the stored fluid in marine environments. Such a storage tank may be capable of storing large volumes (e.g., 100,000 cubic meters (m3) or more) of fluids and easily fabricated.
Other related material may be found in at least U.S. Pat. No. 3,332,386; U.S. Pat. No. 3,759,209; U.S. Pat. No. 3,941,272; U.S. Pat. No. 5,727,492; U.S. Patent Pub. No. 2004/0172803; U.S. Patent Pub. No. 2004/0188446; U.S. Patent Pub. No. 2005/0150443; Hermundstad, et al., “Hull Monitoring,” Society of Petroleum Engineers, Paper No. 61454-MS, pp. 1231-1240, Jun. 26, 2000; and Vandiver, et al., “The Effect of Liquid Storage Tanks on the Dynamic Response of Offshore Platforms,” Society of Petroleum Engineers, Paper No. 7285-PA, pp. 1-9, October 1979.